Wireless terminal devices such as mobile phones must meet various requirements. As the physical size of the devices gets smaller, their size imposes their own requirements e.g. on the device structure, and particularly on the ground structure of the device. The antenna is a very important part of a terminal device operating at high frequencies, since it converts the signal received from the transmitter into electromagnetic waves and the electromagnetic waves into the signal to be received.
The antenna has to be capable of receiving and sending signal as well as possible under all circumstances. The frequency and output of a signal passing through the antenna may vary even to a great extent depending on the device, system and conditions, so as concerns the functioning of the device, it is substantial that the antenna operates under all circumstances as well as possible. In practice all antennas need a properly operating ground arrangement in order to efficiently function as emitters. The ground arrangement is herein also referred to as the “antenna ground”, which is generally used to refer to the earth current arrangement that is used to achieve a low-impedance coupling to the ground potential or to the common reference point.
In practice, interfering signals are received into the antenna, which render difficult the normal functioning of the device, at least to some extent. The devices in use themselves cause interfering signals that may e.g. harm the operation of the components in the device.
Devices operating at a radio frequency cause electric and magnetic fields while in use. Such a place or point at which there is, or at which there each time is created, a concentration of electric or magnetic field is called a so-called hot spot. From the distribution of currents induced by the fields it is possible to approximately calculate the specific absorption rate (SAR), which indicates the output absorbed (e.g. by the bodily tissue) per mass unit. By means of the ground arrangement of the antenna it is possible to affect the magnitude of the SAR value.
In addition to the output, the SAR value depends e.g. on the frequency, the distance of the antenna in relation to the user, the service position of the phone and the antenna type. While the SAR values of the devices on the market are very small, there is an attempt to design the new devices in such a manner that one could get the SAR values smaller than before.
It is known to attach to the terminal devices various additional components that help extend the antenna ground of the terminal device when necessary, which means that the antenna amplification is bigger. The user has had e.g. to detach an additional component from the terminal device in order to introduce the ground extension. The aforementioned solution is therefore cumbersome in use.
The objective of the invention is to improve the features of the device as concerns the above-mentioned facts and to achieve a method more workable than before for controlling and changing the ground configuration of the device. In addition, the objective is to achieve a solution by means of which it is possible to direct the earth currents to a place as optimal as possible in such a manner that the created field concentrations are as far as possible from the user, in which case it is possible to obtain a small SAR value.